Red electroluminescence of ruthenium sensitizer functionalized by sulfonate anchoring groups

Article abstract of DOI:10.1039/c3dt52949e

We have synthesized five novel Ru(II) phenanthroline complexes with an additional aryl sulfonate ligating substituent at the 5-position [Ru(L)(bpy)₂](BF₄)₂ (1), [Ru(L)(bpy)(SCN) ₂] (2), [Ru(L)₃](BF₄)₂ (3), [Ru(L)₂(bpy)](BF₄)₂ (4) and [Ru(L)(BPhen)(SCN)₂] (5) (where L = 6-one-[1,10]phenanthroline-5- ylamino)-3-hydroxynaphthalene 1-sulfonic, bpy = 2,2′-bipyridine, BPhen = 4,7-diphenyl-1,10-phenanthroline), as both photosensitizers for oxide semiconductor solar cells (DSSCs) and light emitting diodes (LEDs). The absorption and emission maxima of these complexes red shifted upon extending the conjugation of the phenanthroline ligand. Ru phenanthroline complexes exhibit broad metal to ligand charge transfer-centered electroluminescence (EL) with a maximum near 580 nm. Our results indicated that a particular structure (2) can be considered as both DSSC and OLED devices. The efficiency of the LED performance can be tuned by using a range of ligands. Device (2) has a luminance of 550 cd m^-2 and maximum efficiency of 0.9 cd A^-1 at 18 V, which are the highest values among the five devices. The turn-on voltage of this device is approximately 5 V. The role of auxiliary ligands in the photophysical properties of Ru complexes was investigated by DFT calculation. We have also studied photovoltaic properties of dye-sensitized nanocrystalline semiconductor solar cells based on Ru phenanthroline complexes and an iodine redox electrolyte. A solar energy to electricity conversion efficiency (η) of 0.67% was obtained for Ru complex (2) under standard AM 1.5 irradiation with a short-circuit photocurrent density (J_sc) of 2.46 mA cm ^-2, an open-circuit photovoltage (V_oc) of 0.6 V, and a fill factor (ff) of 40%, which are all among the highest values for ruthenium sulfonated anchoring groups reported so far. Monochromatic incident photon to current conversion efficiency was 23% at 475 nm. Photovoltaic studies clearly indicated dyes with two SCN substituents yielded a higher J_sc for the cell than dyes with a tris-homoleptic anchor substituent. the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt52949e

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Red electroluminescence of ruthenium sensitizer
functionalized by sulfonate anchoring groups†
Cite this: Dalton Trans., 2014, 43,
9202
Hashem Shahroosvand,*a Parisa Abbasi,a Ezeddin Mohajeranib and
Mohammad Janghourib
We have synthesized five novel Ru(II) phenanthroline complexes with an additional aryl sulfonate ligat-
ing substituent at the 5-position [Ru(L)(bpy)2](BF4)2 (1), [Ru(L)(bpy)(SCN)2] (2), [Ru(L)3](BF4)2 (3),
[Ru(L)2(bpy)](BF4)2 (4) and [Ru(L)(BPhen)(SCN)2] (5) (where L = 6-one-[1,10]phenanthroline-5-ylamino)-
3-hydroxynaphthalene 1-sulfonic, bpy = 2,2’-bipyridine, BPhen = 4,7-diphenyl-1,10-phenanthroline), as
both photosensitizers for oxide semiconductor solar cells (DSSCs) and light emitting diodes (LEDs). The
absorption and emission maxima of these complexes red shifted upon extending the conjugation of the
phenanthroline ligand. Ru phenanthroline complexes exhibit broad metal to ligand charge transfer-
centered electroluminescence (EL) with a maximum near 580 nm. Our results indicated that a particular
structure (2) can be considered as both DSSC and OLED devices. The efficiency of the LED performance
can be tuned by using a range of ligands. Device (2) has a luminance of 550 cd m−2 and maximum
efficiency of 0.9 cd A−1 at 18 V, which are the highest values among the five devices. The turn-on voltage
of this device is approximately 5 V. The role of auxiliary ligands in the photophysical properties of Ru com-
plexes was investigated by DFT calculation. We have also studied photovoltaic properties of dye-sensitized
nanocrystalline semiconductor solar cells based on Ru phenanthroline complexes and an iodine redox
electrolyte. A solar energy to electricity conversion efficiency (η) of 0.67% was obtained for Ru complex
(2) under standard AM 1.5 irradiation with a short-circuit photocurrent density (Jsc) of 2.46 mA cm−2, an
open-circuit photovoltage (Voc) of 0.6 V, and a fill factor (ff) of 40%, which are all among the highest
values for ruthenium sulfonated anchoring groups reported so far. Monochromatic incident photon to
current conversion efficiency was 23% at 475 nm. Photovoltaic studies clearly indicated dyes with two
SCN substituents yielded a higher Jsc for the cell than dyes with a tris-homoleptic anchor substituent.
Received 18th October 2013,
Accepted 10th March 2014
DOI: 10.1039/c3dt52949e
www.rsc.org/dalton
perties of sensitizer molecule structures is one of the most
important tasks for developing a high performance solar cell.
1. Introduction
A global challenge is to capture and utilize solar energy for sus- Ruthenium polypyridine dyes have received much attention
tainable development and to use the produced energy for low- due to their outstanding performance as sensitizers in DSSCs,
cost, low-consumption lighting solutions. In this respect dye- reaching efficiencies of up to 11%.12–15 In this respect, the
sensitized solar cells (DSSCs)1–6 and organic light-emitting nature of anchoring groups is of utmost importance because it
diodes (OLEDs)7–10 represent valuable examples of both greatly influences the transport and adsorption of the dye onto
technologies, which can also be integrated into a unique the semiconductor surface. Moreover, knowledge of the nature
photonic device. In light harvesting, the first step in such con- and number of binding modes and of the binding geometry
version, the choice of a photosensitizer capable of efficient are crucial factors for the study of interfacial injection pro-
visible-light absorption is crucial. Therefore, much effort has cesses, to improve the electrochemical devices, and to design
focused on the design and synthesis of various dyes, including new sensitizers. A number of Ru-polypyridine sensitizers with
metal complexes and organic dyes.11 Understanding the different anchoring moieties have been reported. Chun-Hui
relationship between the solar cell performance and the pro- Huang et al. indicated that linking attaching groups such as
−
–RSO3 to hemicyanine dyes results in successful adsorption
onto the surface of TiO2 and shows good charge-transfer
aChemistry Department, University of Zanjan, Zanjan, Iran.
properties.16–18 Although the formation of an ester-like linkage
E-mail: shahroos@znu.ac.ir
bLaser and Plasma Research Institute, Shahid Beheshti University, Tehran, Iran
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
between carboxylic acid groups of the corresponding bipyri-
dine ligands and TiO2 nanocrystallites is now well
established,19–22 there is a lack of experimental data on the
c3dt52949e
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kind of chemisorption of dyes bearing sulfonic acid functional internal standard ferrocene value of −4.8 eV with respect to
groups. Areas of particularly strong interest have been the the vacuum.32,33 The PL spectra of the ruthenium compounds
generation and evaluation of new dyes for intensely lumine- and PVK : PBD were measured in 1,2-dimethylformamide solu-
2+
scent complexes in OLEDs. Since the discovery that Ru(bpy)3
tion. The PL spectra were recorded with an Ocean Optic
is electroluminescent,23,24 a large body of literature has spectrometer USB2000 during 405 nm irradiation. The photo-
appeared aimed at understanding both the ground and excited current–voltage (I–V) characteristics were recorded using a
2+
state properties of Ru(bpy)3 and its derivatives.25,26 With prio- computer-controlled Keithley 2400 source meter under air
rities of electrochemical, photochemical and thermal stabi- mass (AM) 1.5 simulated illumination (100 mW cm−2). The
lity, ruthenium (Ru)-based charge transfer complexes are action spectra of monochromatic incident photo-to-current
receiving considerable attention as an important material conversion efficiency (IPCE) for solar cells were performed using
class for light emitting device study.27,28 By attaching groups a commercial setup for IPCE measurement (PV-25 DYE, JASCO).
such as phosphonato, sulfonato or carboxylato to the bipyri- Full geometric optimization and the calculation of the energetics
dine or phenanthroline moieties, Ru complexes can be linked for all of the structural variables were obtained using the B3LYP/
to biological molecules such as antibodies and DNA, where LanL2DZ method. All calculations and optimizations were per-
they serve as a tag or label for analyses, much like radioactive formed using the Gaussian 03 package.34
or fluorescent labels.29 This has resulted in a wide range of
analytical applications for EL in which Ru polypyridine with
2.2 Preparation of EL devices and testing
anchoring group plays a key role. Moreover, the demonstration
of simultaneous multi-color luminescence from metal com-
plexes assemblies using two different strategies: the detection
of a picosecond-scale luminescence component from ruthe-
nium polypyridine complexes arising from a non-thermalised
excited state; and generation of long lived charge-separated
states of up to 1 second in a donor–chromophore–acceptor
triad. High-brightness and high-efficiency emissions with low-
driving voltage of Ru-complex-based solid and solution-state
EL cells have been achieved.30,31 Finally, Ru polypyridine com-
plexes with anchoring group exhibit a variety of low-lying elec-
tronically excited states (p–p*, d–d*, d–p* and MLCT), which
make them good candidates for both dye sensitized solar cells
and light emitting diodes. This paper concerns the synthesis
of five novel ruthenium polypyridine complexes by using a
π-extended phenanthroline ligand. Our efforts are focused on
the finding a dye that could be used as both a dye sensitized
solar cell and light emitting diodes.
The structure of the fabricated device is as follows:
ITO/PEDOT:PSS(90 nm)/PVK:PBD(70 nm)/Al(200 nm) and
ITO/PEDOT:PSS(90 nm)/PVK:PBD:ruthenium complex (70 nm)/
Al(200 nm), as shown in Fig. 1.
PVK as a hole-transporting material and PBD as an elec-
tron-transporting material were doped with ruthenium com-
pounds. Glass substrates, coated with ITO (sheet resistance of
70 Ω m−2), were used as the conducting anode. The ratio of
ruthenium complexes for each type were 10 wt% in PVK : PBD
(100 : 40). PEDOT:PSS(poly(3,4-ethylenedi-oxythiophene):poly-
(styrenesulfonate)) was used as a hole injection and transport-
ing layer.
All polymeric layers were successively deposited onto the
ITO coated-glass by using a spin-coating process from the solu-
tion. A metallic cathode of Al was deposited on the emissive
layer at 8 × 10−5 mbar by thermal evaporation. The PEDOT:PSS
was dissolved in DMF, spin coated onto ITO and was held in
an oven at 120 °C for 2 hours after deposition. PVK:PBD and
ruthenium complexes with a ratio of 10 : 4 : 1 were blended in
DMF, and then spin coated and baked at 80 °C for 1 hour. The
thickness of the polymeric thin film was determined by a
Dektak 8000. The EL intensity and spectra were measured with
an Ocean Optic USB2000 under ambient conditions. In
2. Experimental
2.1 Materials and instruments
All chemicals and solvents were purchased from Merck or
Aldrich and used without further purification. IR spectra were
recorded on a Perkin-Elmer 597 spectrometer. 1H-NMR spectra
were recorded by use of a Bruker 250 MHz. Electrochemical
measurements were made in DMF using a model SAMA 500
potentiostat. Electrochemical experiments were carried out in
an airtight single-compartment cell by using platinum working
and counter electrodes and a silver spiral as a quasi-reference
electrode. The supporting electrolyte was 0.1 M [Bu4N]BF4.
Ferrocene was added as an internal standard after each set of
measurements, and all potentials reported were quoted with
reference to the ferrocene–ferrocenium (Fc/Fc+) couple at a scan
rate of 100 mV s−1. The oxidation (Eox) and reduction (Ered
)
potentials were used to determine the HOMO and LUMO
energy levels using the equations EHOMO = −(Eox + 4.8) eV and
ELUMO = −(Ered + 4.8) eV, which were calculated using the Fig. 1 The layer arrangement of the Ru-based LED-device.
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addition, a Keithley 2400 sourcemeter was used to measure the desired ligand. Yield 82%; m.p. = 232 °C; 1H NMR
the electrical characteristics of the devices.
(250 MHz, DMSO-d) (see Scheme S1, ESI†for proton labeling):
δ = 10.22 (br.), 9.68 (s, 1H, H4), 9.08 (d, 2H, H3), 8.73 (d, 2H,
H5), 8.13 (d, 2H, H1), 7.66 (t, 2H, H6). CHN, anal. calc. for (L),
(C22H13N3O5S): C, 61.250; H, 3.034; N, 9.740. Found: C, 61.208;
H, 2.980; N, 9.712%. This component will be denoted by (L)
(Scheme 1).
2.3 Fabrication of DSSC
To prepare the DSSC working electrodes, the transparent con-
ductive oxide (TCO) glass used as current collector was first
cleaned in a detergent solution using an ultrasonic bath for
15 min, and then rinsed with water and ethanol. The meso-
scopic TiO2 film used as photoanodes consisted of layers of
TiO2. TiCl4 treatment was applied to the prepared TiO2 films.
The sintered TiO2 films were dipped in an aqueous solution of
50 mM TiCl4 at 70 °C for 30 min, and sintered again for
30 min at 450 °C (10 µm thick transparent layer of 25 nm TiO2
anatase nanoparticles and a 5 µm thick scattering layer of
400 nm anatase TiO2 particles). The sintered TiO2 electrode
was then immersed for 16 h in a 3 × 10−4 M DMF solution of
the dyes. The counter electrode was prepared on a bare FTO
substrate. The Pt precursor was prepared by H2PtCl6, and
spread on the front side of the FTO glass. The electrolyte con-
sisted of 0.1 M LiI, 0.03 M I2, and 0.5 M 4-tert-butylpyridine in
3-methoxypropionitrile.
Homoleptic and heteroleptic Ru(II) complexes were syn-
thesized by standard procedures according to Scheme 2.
2.4.2 Synthesis of [RuII(bpy)2(L)](BF4)2 (1). For complex (1),
in a 100 ml round-bottomed flask with three vertical necks
equipped with a gas-inlet and a condenser combined with gas-
outlet, under nitrogen was introduced 1 equiv. of (L) 0.07 g
(0.16 mmol) and 1 equiv. of trichlororuthenium(III) 0.0364 g
(0.16 mmol). The necessary volume of degassed ethanol to dis-
solve the solids was added and the mixture was refluxed for
4 h. To this intermediate complex was added 2 equiv. of 2,2-
bipyridine (bpy; 0.054 g, 0.32 mmol) in the minimum volume
of dry and degassed ethanol. Then, excess NaBF4 (0.06 g) was
added and the solution was refluxed for a further 4 h. After
this time, the reaction mixture was allowed to cool, and the
precipitate was filtered and washed with deionized water,
ethanol and diethyl ether. This product was further purified by
column chromatography using alumina as the column support
and acetonitrile–methanol (2 : 1, v/v) as the eluent. The major
brown band was collected and corresponds to the desired
2.4 Synthesis of ligand and complexes
2.4.1 Synthesis of 6-one-[1,10]phenanthroline-5-ylamino-3-
hydroxynaphthalene-1-sulfonic acid (L). The 1,10 phenanthro-
line-5,6-dione ligand was prepared according to the literature
method.35 For synthesis of (L), 1,10-phenanthroline-5,6-dione
(100 mg, 0.48 mmol) and 5-ylamino-3-hydroxynaphthalene-1-
sulfonic acid (0.114 g, 0.48 mmol) were taken in ethanol
(50 ml) and refluxed for 4 h. After solvent evaporation at room
temperature, a dark red powder appeared. This product was
future purified by column chromatography using alumina as
the column support and chloroform–methanol (6 : 10, v/v) as
the eluent. The major band was collected and corresponds to Scheme 1 Synthesis procedure of (L).
Scheme 2 Schematic representation of different synthetic approaches to Ru(II)-complexes (1–5).
9204 | Dalton Trans., 2014, 43, 9202–9215
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1
complex. Yield 62%; m.p. >250 °C; H NMR (250 MHz, DMSO-
2.4.6 Synthesis of [RuII(BPhen)(L)(SCN)2] (5). Complex (5)
d): δ = 10.35 (br.), 9.68 (s, 1H, H4), 9.06 (d, 2H, H3), 8.90 (d, was prepared using the same procedure as for complex (2)
2H, H5), 8.82 (q, 4H, Hd, Hd′), 8.25–8.31 (m, 6H, Ha, Ha′, H1), except 1 equiv. bpy was replaced by 1 equiv. BPhen 0.032 g
8.19 (t, 4H, Hb, Hb′), 8.1–7.93 (m, 4H, Hc, Hc′), 7.85 (t, 2H, (0.016 mmol). The product was further purified by column
H2), 7.68 (t, 2H, H6); CHN, anal. calc. (C42H29N7O5SB2F8Ru): chromatography using alumina as the column support and
C, 51.233; H, 6.015; N, 16.654. Found: C, 51.276; H, 6.042; N, acetonitrile–methanol (2 : 1, v/v) as the eluent. The major dark
16.612%.
brown band was collected and corresponds to the desired
2.4.3 Synthesis of [RuII(bpy)(L)(SCN)2] (2). [RuII(bpy)(L)- complex. Yield 40%; m.p. >250 °C; H NMR (250 MHz, DMSO-
(SCN)2] (2) was prepared starting from RuCl3·xH2O (0.036 g, d): δ = 10.35 (br.), 9.82 (s, 1H, H4), 9.08 (q, 4H, H3, Hk), 8.88
0.16 mmol), bpy (0.027 g, 0.16 mmol) and L (0.036 g, (d, 2H, H5), 8.76 (q, 4H, Hi, H1), 8.66–8.42 (m, 6H, Hj, Hj,
0.12 mmol) using the same procedure as described for the syn- Hm, Hm′), 8.12 (t, 4H, Hn, Hn′), 7.87 (q, 4H, Hl, Hl′),
thesis of [RuII(bpy)2(L)](BF4)2 (1) except an excess of KSCN 7.73–7.67 (m, 4H, H2, H6). CHN, anal. calc. (C48H29N7O5Ru):
instead of NaBF4 was added to the brown mixture (10 times C, 61.012; H, 4.034; N, 10.684. Found: C, 61.077; H, 4.078;
the stoichiometric amount) and the mixture refluxed for 4 h. N, 10.602%.
1
The reaction mixture was allowed to cool to room temperature
before being filtered through a sintered glass crucible. The
fine black powder in suspension was recovered by centrifu-
gation at the speed of 5000 rev min−1 for 15 min. Sub-
sequently, the products were washed with water, ethanol and
3. Results and discussion
3.1 Characterization of complexes
diethyl ether three times and dried in air at room temperature.
The UV-Vis absorption spectra of the ligand (L) and the com-
This product was further purified by column chromatography
plexes (1–5) in acetonitrile solution are given in Fig. 2.
using alumina as the column support and acetonitrile–metha-
It can be seen that the absorption bands of ligands appear
nol (2 : 1, v/v) as the eluent. The major dark brown band was
in the ultraviolet region at around 220–310 nm due to π–π*
collected and corresponds to the desired complex. Yield: 52%;
transitions of the aromatic rings. Free (L) shows three bands at
230, 265 and 305 nm. Moreover, three bands at 218, 260 and
310 nm were observed for bpy. As shown in Fig. 2, the absorp-
tion spectra of the complexes show intense bands in the ultra-
violet region, which can be attributed to the spin allowed π*
transitions of the (L) and bpy or BPhen ligands. It shows that
after coordination the absorption bands reveal a red-shift
owing to the formation of the rigid conjugated system after
m.p. >250 °C; 1H NMR (250 MHz, DMSO-d): δ = 10.39 (br.),
9.71 (s, 1H, H4), 9.10 (d, 2H, H3), 8.82 (d, 2H, H5), 8.33–8.25
(m, 4H, Hd, Hd′, H1), 8.15 (q, 2H, Ha, Ha′), 7.90 (t, 2H, Hb,
Hb′), 7.80 (t, 2H, Hc, Hc′), 7.70 (m, 4H, H2, H6); CHN, anal.
calc. (C34H21N7O5S3Ru): C, 53.112; H, 3.654; N, 13.281. Found:
C, 53.134; H, 3.602; N, 13.312%.
2.4.4 Synthesis of [RuII(L)3](BF4)2 (3). A mixture of
RuCl3·xH2O (0.021 g, 0.14 mmol) and (L) (0.1 g, 0.42 mmol) in
coordination, which confirms that the UV spectra of the com-
50 ml ethanol was refluxed under a nitrogen atmosphere for
plexes reflect essentially absorption of the ligands.
4 h. Then, excess NaBF4 (0.6 g) was added and the solution
The broad absorption bands from 400 nm extending to
was refluxed for a further 4 h. The resulting precipitate was fil-
600 nm correspond to both spin allowed and spin forbidden
tered and washed with ethanol and diethyl ether. The isolated
solid was recrystallized from methanol–diethyl ether, after
which it was further purified on an alumina column, using
acetonitrile–methanol (2 : 1, v/v) as the eluent. Yield 41%; m.p.
1
>250 °C; H NMR (250 MHz, DMSO-d): δ = 10.40 (br), 9.70 (s,
2H, H4), 9.0 (d, 4H, H3), 8.85 (d, 4H, H5), 8.72–8.67 (m, 6H,
Ha, Ha′, H1), 8.20 (q, 2H, Hd, Hd′), 8.09 (t, 2H, Hb, Hb′),
7.82–7.71 (m, 6H, H2, Hc, Hc′), 7.65 (t, 4H, H6); CHN, anal.
calc. (C66H39N9O15 S3Ru): C, 54.187; H, 3.767; N, 9.381. Found:
C, 54.202; H, 3.702; N, 9.402%.
2.4.5 Synthesis of [RuII(bpy)(L)2](BF4)2 (4). Complex (4)
was prepared using the same procedure as for complex (1)
except using 2 equiv. (L) (0.1 g, 0.42 mmol) and 1 equiv. bpy
(0.0018 g, 0.21 mmol). The isolated solid was recrystallized
from methanol–diethyl ether, after which it was further puri-
fied on a alumina column, using acetonitrile–methanol (2 : 1,
v/v) as the eluent. Yield 51%., m.p. >250 °C. 1H NMR
(250 MHz, DMSO-d) δ: 10.84 (br), 9.70 (s, 3H, H4), 9.0 (d, 6H,
H3), 8.72 (d, 6H, H5), 8.72–8.34 (m, 6H, H1), 8.0–7.73 (m, 12H,
H6, H2); CHN, anal. calc. (C54H34N8O10S2B2F8Ru): C, 53.635;
H, 3.294; N, 9.165. Found: C, 53.605; H, 3.312; N, 9.202%.
Fig. 2 Absorption spectra of ruthenium complexes (1–5) in acetonitrile
solution (10−5 M).
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Fig. 4 Simplified energy level diagram and the emission states for Ru
polypyridine complexes. (a) Normal 3MLCT emission; (b) 3ILCT/3LLCT
emission.
Fig. 3 PL spectra of ruthenium complexes (1–5) in DMF solution
10−5 M (λexc = 405 nm).
characteristics of 3MLCT states, with regard to energy and
excited-state lifetime.36
The different photophysical processes in the ruthenium
metal to ligand charge transfer (MLCT) transitions. The inten- polypyridine complexes are summarized in Fig. 4. It is useful
sity of the spin forbidden bands arises from the strong spin to note such a dpRu(II)→p*L/bpy based 3MLCT transition band in
orbit coupling of ruthenium(II), which causes mixing with the region 450–550 nm has been observed earlier by many
higher-lying spin allowed transitions.
researchers for various Ru(II)–polypyridine complexes.40 Also,
The PL characteristics of the ruthenium complexes are the combination of 3MLCT and 3LLCT absorption bands was
shown in Fig. 3. The PL emission spectra of the complexes in responsible for the broad absorption spectrum in the longer
3
3
DMF solution were obtained by exciting with a wavelength of wavelength region. In normal MLCT emission, the MLCT is
405 nm. As shown in Fig. 3, the Ru complexes emit yellowish- populated by internal conversion (IC) between the S1 and the
green light at room temperature with emission maxima (lem) at 1MLCT and a subsequent MLCT–3MLCT ISC (upon excitation
1
530 and 570 nm. The broad spectra indicates that the emissive at 400 nm) or by direct inter system crossing (ISC) between
1
excited states of the complexes have more characteristics of the photoexcited MLCT (upon excitation at 525 nm) and the
3
3
MLCT rather than ligand to ligand charge transfer (π→π*). In 3MLCT. The crossing of MLCT states to lower energy ILCT or
Fig. 3, a red shift and a full-width at half-maximum (FWHM) 3LLCT excited states creates the inherent lower energy of 3ILCT
enhancement of the emission peaks were observed for or LLCT states with respect to the MLCT excited state, which
3
3
complex (2) as compared to the other complexes. Appending leads to photoluminescence in the near-IR region.
aryl sulfonato functionalities also profoundly increases elec-
The internal conversion from 3MLCT to 3ILCT or/and
tronic delocalization in the excited state of conjugated 3LLCT states is reported as a dominant fast phase emission
systems, inducing significant red-shifts in emission wave- quenching of the MLCT state in the sub-ns time domain.41 It
3
length, and increasing emission intensity. Interestingly, the is useful to note that the ligand localized CT states (3ILCT or
extended conjugation of the phen ligand at a particular posi- 3LLCT) emission is too low to observe in emission spec-
tion produced dual emission that was readily resolved in both troscopy at room temperature.42 Earlier, the 3MLCT→3ILCT
energy and lifetime.30,36 In contrast to compounds (2) and (5), internal conversion is reported by Charlot et al43 to be as fast
complex (3) exhibited lower luminescent properties than (2) as ∼100 ps. Generally, ruthenium polypyridine complexes
and (5); the additional conjugation to the metal center did not exhibit long 3MLCT lifetimes from millisecond to micro-
seem to have any impact on the coordination complex when second. As an example, a family of Ru(II) complexes containing
three phen aryl sulfonate ligands were coordinated to ruthe- substituted 1,10-phenanthroline ligands with extended conju-
nium. To our knowledge, this is not the first report of surpris- gation display two simultaneously emissive 3MLCT excited
ing substitution effects at this position of the phen ligand. states in solutions at room temperature, originating from bpy-
Wallace et al. discovered that substitution at the 4,7-positions based (t = 0.9 to 2.2 ms) and longer lived phen-based (t = 3.7 to
had a far greater impact than any other substitution position 11.5 ms) excited states.44 Therefore, the assignment of 3ILCT
on the MLCT energy at room temperature for ruthenium com- and 3LLCT to emission bands at centered 570 nm will be ruled
plexes, and reported low-temperature dual emission of the out and 3MLCT→S1 will be reasonable.
“distinct orbital type” for these systems.37–39 In contrast,
Yitzhak Tor clearly indicated that both states display the (1–5) are summarized in Table 1. Generally, in the IR spectrum
The main absorption peaks in the IR spectra of complexes
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Table 1 The main absorption peaks in the IR spectra of complexes (1–5), (cm−1
)
Dye
ν(SO3)sym
ν(SO3)asym
ν(O–H)
ν(SCN)
ν(C–H)
ν(CvN)imin
ν(CvO)
L
1090
1069
1043
1036
1043
1052
1290
1299
1255
1226
1298
1241
3400
3400
3414
3406
3412
3410
—
—
2097
—
2088
—
2922
2918
2918
2919
2920
2919
1704
1699
1689
1707
1728
1692
1684
1625
1632
1632
1632
1626
(1)
(2)
(3)
(4)
(5)
of phen, strong bands were observed in frequency region BPhen and two (L) units. The difference between the chemical
between 1400 and 1650 cm−1, the one band occurring at shift of the free ligands and the complexes indicates that the
1505 cm−1, the second appearing at 1590 cm−1 and the third coordination of ligands to metal has occurred. The cyclic vol-
band at 1423 cm−1
.
tammograms of complexes (1–5) show features characteristic
In the present study, the IR spectrum of (L) shows a ν(C–N) for ruthenium–polypyridine complexes, with a reversible metal
band related to phen at 1422 cm−1, indicating the partial based oxidation (RuIII/RuII) at positive potentials from +0.9 (V)
double bond character. The ring frequencies associated with to +1.4 (V) and four quasi-reversible reductions at negative
phen are observed at 1497 and 1601 cm−1 for (L). The ν(C–H) potentials from −1.1 (V) to −1.9 (V) (ESI,† S2). The first
of phen bands appear at 2876, 2946 and 2965 cm−1. In the aryl reduction wave is due to the reduction of the sulfonyl ligand,
sulfonyl ligand (L), bands at 1290, 1090, 684 and 561 cm−1 are and is followed, at more negative potentials, by three succes-
derived from the presence of SO3. The vibration band at sive electron reduction of the bpy or phen ligands.46 The
1030–1090 cm−1 is characteristic of the vibrations of the sulfur LUMO and HOMO of the ground state of the dyes (1–5) are
atoms linked to the aromatic cycle and of SvO symmetric about 2.3–3.8 and 3.0–5.4 V versus NHE. These energy levels of
vibration.45 The asymmetric SvO vibration is observed at LUMO and HOMO are able to inject energetically electrons
1220–1290 cm−1. It is important to note that the disappear- into the conduction band of TiO2 and accept electrons from
ance of the ν(NH2) stretching frequency of the amine group iodide ions, respectively.
and appearance of ν(NH) stretching at 3445 cm−1 in the IR
spectrum of (L) reveals the formation of (L). The region of par-
3.2 EL characterization
ticular interest is between 1800 and 1000 cm−1, as the various
Fig. 5 shows the EL spectra of PVK:PBD and ruthenium com-
plexes (1–5) in the PVK:PBD blend. An emissive layer without
any ruthenium complex was fabricated to record the PVK:PBD
EL spectra and to find a relationship between the EL spectra of
C–O stretching bands that are found here indicate the types of
C–O bonding present in the molecule. This region is compli-
cated, with vibrations of the bpy framework and sulfonate
groups all contributing to the spectrum. In the IR spectra of
complexes (1–5), strong peaks at 2918–2922 cm−1 for com-
plexes (1–5) were assigned to the (C–H) group, and a strong
absorption at 1226–1290 cm−1 for all complexes (1–5) corres-
ponds to the ν(SO3) vibration of the sulfonate group. The
NCS– group has two characteristic modes, ν(NC) and ν(CS),
which are frequently considered as diagnostic with respect to
the coordination mode of the ambidentate NCS ligand. The IR
spectra of complexes (4) and (2) show an intense absorbance at
2088 cm−1 and 2097 cm−1, respectively, for ν(NC) and 780 and
785 cm−1, respectively, for ν(CS) due to the N-coordinated NCS
ligand. There are a number of bands at lower energy (to
1000 cm−1) in the dyes that contain both C–C, C–N and S–O
stretching and S–O deformation character. Further infor-
mation on the complexes was obtained from 1H-NMR spec-
troscopy. 1H-NMR spectra of all complexes show all peaks of
synthesized ligand (L), bpy, phen and BPhen in the region
7.5–10.8 ppm, which indicates the presence of the ligands in
the complexes. The signal integration for complex (1) reveals
the incorporation of one (L) and two bpy units. For complex
(2), two (L) units and one bpy unit were found by signal inte-
gration. Three and two (L) units were revealed by signal inte-
gration for complexes (3) and (4), respectively. The signal
integration for complex (5) reveals the incorporation of one
Fig. 5 EL spectra of PVK:PBD and ruthenium complexes (1–5) in the
PVK:PBD blend.
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the ruthenium compounds and PVK:PBD EL in order to separ-
ate it from the emission of ruthenium complexes (Fig. 5). In
the EL spectra, the ruthenium complexes show a long red shift
compared to the PVK:PBD EL spectrum, demonstrating that
effective energy transfer is taking place in the emissive layer.
As the driving voltage is biased to electrodes, the electrons
and holes begin to inject into the bulk. Some of the electrons
and holes under the electromagnetic forces emit light with
their mobility between HOMO and LUMO energy levels of
layers.
To achieve a good balance of holes and electrons, both hole
and electron-transporting functions (PVK:PBD) were incorpor-
ated into a single Ru emitter. It should be noted that our
results show that a high ratio of PBD leads to lower stability of
the device due to the decrease in the ratio of PVK.
However, the ruthenium devices made without PBD in the
blend layer are weakly electroluminescent and PVK is the
dominant emitter. Finally, the efficient ratio of 10 : 4 : 1 for
PVK : PBD : dye was suggested, which reduces the risk of
excimer formation and increases the stability of the device.
The use of a PEDOT:PSS interlayer helps also better hole injec-
tion into the emissive layer and results in increasing the
current and the probability of exciton formation.
The formation of Forster transfer is possible in Ru com-
plexes since there is overlap between the absorption spectra of
dyes and emission spectra of PVK:PBD. Moreover, the arrange-
ment of layers caused the Forster transfer of energy from PVK:
PBD host to ruthenium complexes (Fig. 6). This arrangement
indicates that the NCS ligand can directly affect the position of
the HOMO energy level, in which the HOMO is delocalized
over the NCS ligands. As shown in Fig. 6, these arrangements
of energy levels also allow ruthenium complexes between the
electron transport layer (PBD) and hole injector layer (PVK). As
shown in Fig. 5, device (2) gives the highest EL efficiency com-
pared with other devices. The maxima of EL for Ru complexes
Fig. 7 Current versus applied voltage for devices (1–5).
(1–5) were centered at 580 nm. The electroluminescence spec-
trum results in CIE color coordinates of x = 0.51 and y = 0.41
for dye (2), corresponding to red emission. Fig. 7 shows the
variation of current with respect to voltage increase. As shown
in Fig. 7, all devices show rapid growth in current at voltages
higher than 8 V. It means that at this voltage the potential
barrier for charge injection is overcome by the electric field
exerted between anode and cathode, and helps the rise in the
current. In addition, at specified voltage, for example 15 V,
sample (2) has the highest current compared to others. In
addition, at higher voltages, it is obvious that all samples
depict relatively similar behavior in current change with
voltage.
The device (2) also has a luminance of 550 cd m−2 and
maximum efficiency of 0.9 cd A−1 at 18 V, which are the
highest values among the five devices. Fig. 8 shows the
maximum efficiency (LE) versus applied voltages (V) character-
istics of the Ru devices (1–5). The turn-on voltage of this
device is approximately 5 V. For device (2), the full width at
half-maximum (FWHM), and the correlated color temperature
(CCT) at 18 V were 103 nm and 2145 K, respectively.
The obtained LED performance of sample (2) shows com-
parable data to previously reported Ru complexes to allow
further investigations. Table 2 summarizes the color coordi-
nates in the Commission Internationale de l’Eclairage (CIE
1931) chromaticity chart, the full width at half-maximum
(FWHM) and the correlated color temperature (CCT) for com-
plexes (1–5), (L) and PVK:PBD.
3.3 DFT calculation
DFT calculations were also performed to aid in the interpret-
ation of the role of the ligands in electron transfer mecha-
nisms of the Ru complexes. The optimized structures of the
Fig. 6 Schematic showing the energy levels of device and the dynamic
process of EL emission.
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Fig. 8 The maximum efficiency (LE) versus applied voltages (V) charac-
teristics of Ru devices (1–5).
Fig. 9 Optimized structures of novel ruthenium complexes (1–5).
Table 2 The color coordinates in the Commission Internationale de
l’Eclairage (CIE 1931) chromaticity chart and the full width at half-
maximum (FWHM) and the correlated color temperature (CCT)
such an additional 1-naphthalene carboxylic acid group
decreased their LUMO energy levels considerably relative to
three (1-naphthalene sulfonic acid) groups in complex (3), by
approximately 0.694 eV compared with that of complex (1).
Overall, complex (1) had the highest LUMO level and com-
plexes (2) and (5) had the lowest LUMO level, with a 1.3 eV
difference relative to each other. The relative positions of the
LUMO levels for the donor and acceptor moieties are very
important for charge transport to be effective.47,48 Comparing
the LUMO levels of the donor moieties complexes (1–5) with
those of the acceptor moiety would show that the donor
complex (1) moiety has a LUMO level higher than the acceptor
moiety complexes (2) and (5). Complex (1) has a higher LUMO
level relative to the complex (2) to (5) acceptor moiety. The
donor and acceptor moieties cited above have a LUMO donor–
LUMO acceptor difference that ranges from approximately
0.35–0.62 eV, which is sufficient for charge transport. Fig. 11
shows the HOMO and LUMO orbital spatial orientation of the
novel ruthenium complexes (1–5). Mizuseki et al.49 suggested
that the charge transport was related to the spatial distribution
of the frontier orbitals. The HOMO should be localized in the
donor moiety and the LUMO in the acceptor moiety.50,51
Regarding the HOMO–LUMO gap in the [RuL]2+ complexes, it
appears in the order [Ru(L)(bpy)(SCN)2] (2) ∼ Ru(L)(BPhen)-
(SCN)2](BF4)2 (5) > [Ru(L)(bpy)2](BF4)2 (1) ∼ [Ru(L)3](BF4)2 (3) ∼
[Ru(L)2(bpy)](BF4)2 (4). Considering the HOMO–LUMO gap,
the common features of the UV-Vis absorption spectra are as
follows. (i) The MLCT transitions are hypsochromically shifted
in the presence of the SCN ligand, ranging from 440 nm for (2)
to 485 nm for (3). On the other hand, Π-conjugated ligand is
bathochemically shifted MLCT transition. (ii) Broad bands in
the visible region correspond to the envelope of the Ru–bpy/
Dye
CIE
FWHM (nm)
CCT (K)
(1)
(2)
(3)
(4)
PVK:PBD
(5)
(L)
x = 0.4461, y = 0.4298
x = 0.5117, y = 0.4183
x = 0.5117, y = 0.4183
x = 0.5117, y = 0.4183
x = 0.2707, y = 0.2968
x = 0.4461, y = 0.4298
x = 0.4461, y = 0.4298
158
103
103
103
115
158
158
3054
2145
2145
2145
9471
3054
3054
different donor and acceptor moieties of the ruthenium com-
plexes used in this study are shown in Fig. 9.
Fig. 10 shows the five highest and five lowest molecular
orbital energy levels of the Ru complexes. Note that the LUMO
energy level of the complex (1) containing two bpy groups is
relatively higher than those of the other complexes, with an
estimated value of 0.70–1.3 eV, and that the HOMO energy
level is relatively higher than those of the other complexes
with a difference of approximately 0.605–1.21 eV. This makes
the HOMO–LUMO gaps of the complex (2–5) models smaller
compared to complex (1). The presence of synthesized ligand
(L) in the ruthenium complexes decreases the HOMO–LUMO
gap.
Another interesting feature found in Fig. 10 is the minimal
difference in the bpy ligand of their HOMO and LUMO levels.
The bpy ligand has little influence on the HOMO and
LUMO levels in complex (1), which is in congruence with the
theoretical results of our group. For the acceptor moiety, sulfo-
nic acid is the main functional group, and it is an anchoring
functional group for titanium dioxide, similarly existing in the
structure of the chosen acceptor moieties. The presence of
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Fig. 10 Diagram of the five highest occupied and five lowest unoccupied molecular orbital levels of novel ruthenium complexes (1–5).
Fig. 11 Molecular orbital spatial orientation for complexes (1–5).
BPhen and Ru–L MLCT transitions. It is useful to note that the
In our calculations, complex (1) with the donor having a
status of HOMO and LUMO levels of complexes is important higher LUMO level has its HOMO localized in the donor and
to electron injection in OLED and DSSC. The best match acceptor regions and its LUMO localized in the donor region.
between the LUMO level of the dye and the conduction band Complexes (2) and (5) have (–NCS) groups in their structure,
of TiO2 has been achieved by complex (2). So, we conclude that which caused the localization of the LUMO in the acceptor
the presence of bpy and SCN is the best configuration for the region. The HOMO of structure (1) is localized in the donor
DSSC based sulfonate anchoring group.
moiety and the LUMO is mostly in the same region. The same
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trend was observed for the orbital spatial orientation of ruthe-
nium complex analogues, as shown in Fig. 11. The presence of
ruthenium metal might be responsible for the delocalization
of both the HOMO and LUMO, suggesting the presence of a
metal-to-ligand charge transfer (MLCT) or metal-centered tran-
sitions from both the metal and the ligand p orbitals in the
LUMO.52 Another noteworthy feature in Fig. 10 is the effect of
the presence of (–NCS) groups in the donor moiety of struc-
tures (2) and (5). The electron-donating (–NCS) groups seem to
localize the HOMO on their side. This is very evident in the
structures (2) and (5) containing (–NCS) groups donor moi-
eties. Fig. 11 also shows the HOMO and LUMO levels for the
novel ruthenium complex analogues. The analogues (3) and
(4) have smaller HOMO–LUMO gaps compared with the other
complexes, with a difference of approximately 0.12–0.32 eV.
The results of the calculations revealed that the HOMO and
LUMO values of both the analogues (3) and (4) are roughly the
half HOMO and LUMO values of the structures (2) and (5), as
shown in Fig. 11. It also shows a trend in the combination of
donor moieties with that of the acceptor moieties: if both have
small HOMO–LUMO gaps, the donor–acceptor moieties would
be more likely to have small HOMO–LUMO gaps. This trend is
clearly observed in the donor moieties of structures (2) and (5)
and the structure (3) acceptor moiety, which has the lowest
gap among the moieties studied.
Fig. 12 J–V curves of DSSCs sensitized with complexes (1–5).
Two SCN substituents yielded a higher Jsc for the cell than
the tris ligand (L) substituent. This indicates that the addition
of NCS ligands influences the cell performance. The effect of
anchoring group number on the performance of a DSSC is
critical in achieving high efficiency. Hara et al. found that a
monocarboxylate Ru–phen complex was less efficient than that
obtained using di- or tricarboxylate.53,54
3.4 DSSC measurements
The photovoltaic performances of Ru (1–5) measured under
identical conditions are summarized in Table 3. Fig. 12 shows
comparison of the J–V measurement results of complexes (1–5)
used in the solar cells. Complex (2) showed the highest short
circuit current and fill factor among the dyes. Spectral charac-
teristics of the investigated dyes are similar. It is clear from the
data that dye (2) is the most efficient sensitizer of the all dyes.
Indeed, the efficiencies of the DSSCs prepared with classical
type Ru(bpy)(SCN)2(L), in which (L) is bearing sulfonyl acid,
are significantly (i.e. 10 times) higher than those of the non-
classical dyes. The Jsc value increased in the following order:
(2) > (5) > (1) > (4) > (3).
Yang et al. found that in triphenylamine-based, multiple
rhodanine-3-acetic acid electron acceptor dyes, those which
contained two anchor groups (rhodanine-3-acetic acid) exhibi-
ted superior photovoltaic performance over those with one or
three anchor groups, owing to red shifts and broadening of
absorbance.55 H. Shang et al. indicated the more anchoring
groups the dye has, the lower efficiency the device exhibits.56
Here, the efficiencies of the DSSCs are susceptible to changing
the number of anchoring groups/sulfonyl groups in the dyes.
Increasing the anchor group number resulted in lower short-
circuit current and lower efficiency. Dye (2) showed the highest
Jsc value compared to the other dyes for three main reasons:
the dye solution of a complex with more sulfonyl groups may
contain hydrogen-bonded aggregates on the surface that
prevent penetration into the small pores of the TiO2 surface.
Besides the hydrogen-bonded aggregates, the orientation of
adsorbed sensitizer on the surface may also be responsible for
lower surface concentration of the dye. The other possible
explanation is that the rate of regeneration of dyes with more
anchoring groups with I−/I3− is slower than with complex (2)
since the presence of NCS groups on complex (2) accelerates
the regeneration.57 This will directly impact the number of
electrons being injected, which will consequently influence
the Jsc. Another reason that compounds (1), (3) and (4) fail in
our aim is they have no dye regeneration process as their
HUMO energies lie high above the redox level of electrolyte. As
shown in Fig. 12, the best energy level alignment among
Table 3 Photovoltaic performances of DSSCs sensitized with (1–5) and
Ru sulfonated bipyridine (1–3b and 3e) (radiant power: 27 mW cm−2 (AM
1.0 direct); red/ox electrolyte NMO, LiI (0.5 M), I2 (0.5 mM) and 4-tert-
butylpyridine (100 mM). 1b, [Ru(dsbpy)3]2+; 2b, [Ru(dmbpy)2(dsbpy)]2+
;
3b, [Ru (dsbpy)2Cl2]; 3f, [Ru(dsbpy)2(NCS)2] (where (dmbpy) = 4,4’-
dimethyl-2,2’-bipyridine, and (dsbpy)2 = 2,2’-bipyridine-4,4’-disulfonic
acid)
Dye Jsc (mA cm−2
)
Voc (V) FF (%) η (%) IPCE (%) Ref.
(1)
(2)
(3)
(4)
(5)
1b
2b
3b
3e
0.39
2.46
0.07
0.22
0.54
1.4
1.3
1.3
0.5
0.37
0.6
20
40
12
14
21
34
37
42
36
0.03
0.67
0.003
0.01
0.07
0.9
0.9
1.1
0.4
7
23
2
5
9
—
—
—
—
This work
This work
This work
This work
This work
60
60
60
60
0.28
0.34
0.53
0.52
0.53
0.51
0.52
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Fig. 13 Operating principles and energy level diagram of dye-sensitized
solar cell for complexes (1–5).
Fig. 14 FT-IR spectra of ruthenium complex (1) (red line) and ruthe-
nium complex (1) adsorbed on TiO2 films (black line). Inset: TiO2-
anatase nanostructures extended along the [101] directions sensitized
by dye (1).
HOMO–LUMO of dye, electrolyte and conduction band of TiO2
occurred for dyes (2) and (5). It is very important to note that
when one of the bidentate ligands of [Ru(bpy)3]2+ is replaced
by two NCS ligands, the lower symmetry of the complex com-
promises the degeneracy of the MLCT transitions to lead to a
broader absorption band.58 The installation of the two NCS
ligands also raises the energy of the HOMO, resulting in a
bathochromic shift of the low-energy MLCT band.59
Fig. 15 Possible coordination modes for sulfonate anions in the TiO2
solid state.
Fig. 13 shows the operating principles and energy level
diagram of a dye-sensitized solar cell for complexes (1–5).
FT-IR spectroscopy has been shown to be a powerful tool
for extracting structural information about the molecules
adsorbed onto a TiO2 surface. Fig. 14 shows FT-IR spectra of
ruthenium complex (1) and ruthenium complex (1) adsorbed
on a TiO2 film. The region of particular interest is between
1000 and 1300 cm−1, as the various S–O stretching bands that
are found here indicate the types of S–O bonding that are
present in the molecule.
This region is complicated, with vibrations of the bpy and
aryl ring framework all contributing to the spectra. There are a
number of bands at lower energy (to 1000 cm−1) in dyes and
dyes anchored to TiO2 which contain both C–C and C–N
stretching, S–O stretching and SO3 deformation character.
The other prominent band at 500 cm−1 in anchored Ru
onto TiO2 is due to the (Ti–O) stretching band of the TiO2
surface. For all dyes adsorbed on TiO2, there is an intense,
broad band in the region 1200–1300 cm−1 (overlapping the
highest bpy band), which is assigned to the anti-symmetric
stretch of –SO3−, where the negative charge is delocalized to
give two equivalent (or nearly so) S–O bonds. There is a high-
energy tail to the band indicating some variation in this
difference between νas(SO3) and νs(SO3) (Δν), compared to the
corresponding values in ionic (94 nm), is currently employed
to determine the corresponding mode of the sulfonat group.
The possible binding modes for ruthenium polypyridine con-
taining sulfonato group on a TiO2 surface are shown in Fig. 15.
(1) The unidentate coordination of the sulfonato group
removes the equivalence of the three oxygen atoms, resulting
in ester type bond formation between the sulfonato group and
the TiO2 surface. Unidentate complexes (Fig. 15 (1)) exhibit Δν
values [νas(SO3−) − νs(SO3−)] that are much greater than the
ionic complexes. (2) Chelating complexes (Fig. 15 (2)) exhibit
Δν values that are significantly less than the ionic values. (3)
The Δν values for bridging complexes (Fig. 15 (3)) are greater
than those of chelating complexes, and close to the ionic
values. On the basis of the FT-IR measurements, the difference
between the νas(SO3−) and νs(SO3−) in the Ru complexes
(90–100 cm−1) and the adsorbed Ru complexes (85–110 cm−1
)
suggest that the sulfonato groups are bound to the TiO2
surface via chelating mode (2).
The action spectrum, incident photon-to-electron con-
version efficiency (IPCE) as a function of wavelength, was
measured to evaluate the photoresponse of the photoelectrode
in the whole spectral region. Fig. 16 shows the IPCE spectra of
bonding. The other intense band in this spectrum at
1050–1150 cm−1 is assigned as the symmetric stretch of –SO3
,
−
since it is not assignable to the polypyridine ligands.61–63 The
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and their application in both light emitting diodes and dye-
sensitized solar cells have been reported. This is interesting
because the working mechanism of the above two devices is
nearly inversed. We obtained better efficiency for both DSSCs
and OLEDs prepared with dye (2) in comparison to those with
other dyes under standard conditions. Increased electron
injection is reflected in the increased short-circuit photo-
current density (Jsc) and overall efficiency. DSSC and OLED
studies reveal that ruthenium polypyridine with the NCS
ligand exhibits better current generating capacity than the
ruthenium polypyridine without NCS groups. DFT calculations
also show that complexes (2) and (5) have (–NCS) groups in
their structure, which caused the localization of the LUMO in
the acceptor region. The EL spectra of ruthenium complexes
show a long red shift in visible region related to PVK:PBD
blend. This characteristic, together with favorable elec-
tronic and redox properties as demonstrated by DFT calcu-
lation, optical and electrochemical studies, make Ru(L)(bpy)-
(SCN)2 (2) where (L) = 6-one-[1,10]phenanthrolin-5-ylamino)-3-
hydroxynaphthalene-1-sulfonic attractive for further photo-
electric applications. To our knowledge, this class of dyes has
not been previously reported, and this finding opens a
pathway to designing dyes that both absorb visible light for
energy conversion of solar to electricity and light emission by
further modification of the ligand architecture, which will
improve notably power conversion efficiencies of dye-sensi-
tized solar cells and efficient light emission diodes.
Fig. 16 Photocurrent action spectra of nanocrystalline TiO2 films sensi-
tized by complexes (1–5). The incident photon-to-current conversion
efficiency is plotted as a function of wavelength.
DSSCs based on dyes (1–5). All five dyes can convert visible
light to photocurrent in the range of 400–700 nm. In the range
of 400–500 nm, IPCE maxima of up to 7%, 23%, 2%, 5%, 9%
were achieved for dyes (1–5), respectively. Dye (2) gave broader
spectra and higher photoresponse in the long wavelength
region (>500 nm) than other dyes, which was consistent with
the corresponding absorption spectra on transparent TiO2
film.
According to the photocurrent action spectra, one may
expect higher short-circuit photocurrent densities (Jsc) for dye
(2)-based DSSCs from the overlap integral of IPCE curves with
the standard AM 1.5 solar emission spectrum. Indeed, dye
(2)-based DSSCs give higher Jsc (2.46 mA cm−2) than other
samples (see Fig. 16 and Table 3), which is consistent with the
IPCE data. The sensitizer with more sulforic acid anchoring
groups upon adsorption transfers more protons to the TiO2
surface, leading to the positive shift of the conduction band
edge of TiO2.
Thus, the increasing of the sulforic acid anchoring groups
could lead to increased charge recombination at the TiO2/dye/
electrolyte interface and lower efficiency.64
Further work on ruthenium complexes with other polypyri-
dine ligands is in progress, and we believe that improvement
can be accomplished by a thorough replacement of the
attached functionalized groups to the ruthenium complexes in
both DSSC and OLED fields.
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